The most common non-volatile memory in the market today that is represented by a flash memory, a SONOS (Silicon-Oxide-Nitride-Oxide-Silicon) memory, or the like uses a technology by which a threshold voltage of a semiconductor transistor is changed by storing an electric charge in an insulating film formed on a channel.
In order to realize a large capacity non-volatile memory, a fine element structure has to be developed and used. However, a today's fine processing technology almost reaches a limit of processing or the like. Accordingly, it is difficult to miniaturize even a single semiconductor transistor.
Accordingly, an idea in which the transistor has only a function to select a memory cell used for reading and writing (acts as a switching element) and a structure in which a storage element and the switching element are separated from each other like a DRAM (Dynamic Random Access Memory) is used is considered to realize a large capacity non-volatile memory.
In this case, a variable resistance element whose electric resistance value changes between two or more values when some electrical stimulus is given may be used.
However, an electrical characteristic required when the variable resistance element is used as the storage element is different from that required when the variable resistance element is used as the switching element for performing a switching between the wirings.
Namely, when it is used as the storage element, because the variable resistance element is connected in series to an active element such as a transistor or a diode which selects the storage element, a resistance value of the variable resistance element may be about 1 kΩ in a low-resistance state and it is required to be about 100 kΩ in a high-resistance state. Namely, the resistance value of the variable resistance element has to be changed by two orders between an ON state and an OFF state.
In contrast, when the variable resistance element is arranged between wirings L1 and L2 to configure a switch Sw as shown in FIG. 9, it is required that the resistance value of the variable resistance element in the low-resistance state is equal to a resistance value (for example, 100Ω or less) of the wiring and the resistance value of the variable resistance element in the high-resistance state is 100 MΩ or more to surely cut off a signal.
Accordingly, the variable resistance element has to be produced so that it has an electrical characteristic according to the use.
The various structures are proposed for the variable resistance element. FIG. 10 is a schematic sectional view of a variable resistance element 50 having a metal/metal oxide/metal (hereinafter, referred to as MIM type) structure in which a metal oxide is sandwiched between electrodes. In this variable resistance element 50, a metal oxide film 51 with a resistance change characteristic is arranged between electrodes 52 and 53 that are made of Pt, Ru, or the like. The resistance state is changed by applying a predetermined voltage-current stimulus between the electrodes 52 and 53 from the outside. Further, even when a power supply is disconnected, the resistance state is maintained without being volatilized.
It is disclosed to use NiO (nickel oxide) as the metal oxide film 51 (refer to non-patent document 1). It is proposed to use Ti oxide, Hf oxide, Zr oxide, Zn oxide, W oxide, Co oxide, and Nb oxide as the metal oxide film 51 (refer to patent document 1).
FIG. 11 is a figure showing an example of a current-voltage characteristic of an MIM type variable resistance element. FIG. 11(a) shows an electrical characteristic of the variable resistance element in a case in which a voltage that is equal to or higher than Vt1 is applied to the variable resistance element in the OFF state that is the high-resistance state and the state of the variable resistance element is changed to the ON state that is the low-resistance state. FIG. 11(b) shows an electrical characteristic of a variable resistance element in a case in which a voltage that is equal to or higher than Vt2 is applied to the variable resistance element in the ON state and the state of the variable resistance element is changed to the OFF state that is the high-resistance state.
In such variable resistance element, a current path in the low-resistance state is not distributed and formed over the entire surface of the electrode and is formed locally. A diameter of the current path is about several nm and even in a large case, it is several 10 nm.    [patent document 1] Japanese Patent Application Laid-Open No. 2008-166768    [non-patent document 1] Solid State Electronics vol. 7, pp. 785 to 797, (Solid State Electronics, Vol. 7, P. 785-797, 1964.)